The object of the invention relates to a method for metal casting in green-sand molds, designed so that, while the mold is filled on the inside with the molten metal through the corresponding main casting gutter, there is a simultaneous filling of a secondary critical gutter placed outside the mold, which communicates with the top of said mold, specifically with the reservoir area known as the feeding head, so that once the mold is full the main casting gutter is sealed and immediately after this said feeding head is filled, and thereby the molten metal in said head compensates the contraction of the metal as it solidifies.
A further object of the invention is the sealing device for the main casting gutter, which is also used in the secondary feeding head gutter, with a first closing or sealing of the main casting, gutter when the mold has been filled with molten metal and later sealing of the secondary gutter when the feeding head is filled, all of this being controlled by a probe which detects the filling level of the mold and the feeding head. The aforementioned gutters are sealed by cutting off the sand of the mold and the casting gutter without displacement of sand.
There are well-known processes for obtaining metal parts by casting on sand molds or burrs in which after the burrs are obtained these move along a transportation system and are placed in correspondence with the filling station, which consists of a top closing plate, side plates and a gas evacuation and extraction system, all such that in first place the casting ladle is pressurised and with this the mold is filled, and then the gases contained in the mold cavities are extracted using an evacuation system, so that once the mold is full the inlet gutter for the molten metal is closed and the ladle is depressurised. After these operations are performed and after the mold inlet gutter cools, said mold is removed with the cast metal and a new filling cycle may begin with the next mold.
These molds generally incorporate on their top a feeding head or reservoir which is also filled with molten metal, so that when the metal cools and solidifies there is an amount of reserve metal to compensate the contraction, and thus without this feeding head and the extra supply of molten metal which it provides the part would present surface irregularities and therefore poor quality parts would be produced.
Also known is that in these type of molds the molten metal is poured or fed at the bottom of the mold, so that the feed head is placed at its top.
It has been found that when the hot molten metal is fed on the bottom the mold is filled with molten metal at the bottom, while as the metal approaches the top it is progressively colder, and thus at the very top of the mold, that is at the feed head, the metal is coldest and will therefore solidify first. However, it would be desirable to have the most liquid metal in the feed head so that the metal required call be fed as the metal contracts inside the mold.
One installation for metal casting in green-sand molds is described in Spanish invention patent nxc2x0 9800071 held by this film, which includes a system for sealing or closing the casings gutter comprising a piston driven by a cylinder, with the piston having, ail inner chamber in which a liquid is made to flow, thereby cooling the end and causing a quick solidification of the casting gutter and thus its sealing.
Another method for sealing or closings the casting gutter is described in European Patent no. 0760723, by Georg Fischer Deisa, in which several embodiments are cited for the closings system, one of which consists of a plate with a central orifice which during pouring of the molten metal on the mold is positioned at the inlet gutter, allowing molten metal to pass. After pouring finishes said plate is moved and the inlet gutter closed.
However, this embodiment of a seal has the drawback in that the plate must be preassembled on the mold before casting, which assembly must be very precise so that the plate orifice is aligned with the inlet gutter.
Said European Patent describes, among others, a sealing or closing system based on a piston which is inserted after the casting finishes in a side of the mold, displacing an amount of sand which blocks said inlet gutter.
However, this system has two serious drawbacks, one of which is that a notch must be made on each mold to act as a mark for the insertion of the piston in the collect spot, and the other is that the sealing is achieved by displacement of sand, which in all cases undesirable.
With the method object of the invention the drawback caused by cooling of the molten metal at the top of the mold in order to fill said mold is solved, while with the sealing or closing device for the casting gutter the sand displacement problems and the other problems mentioned in the previous section are also solved.
More specifically, the method metal casting in green-sand molds according to the object of the invention is characterised in that the main casting gutter through which molten metal is fed to the bottom of the mold is prolonged in a secondary vertical gutter on the outside of the mold, which branches from said main casting gutter before reaching the mold, so that as the mold is filled the secondary gutter is also filled by an effect of vessels in communication, with a height of molten metal equal to that in the mould as it is filled, and so that this secondary vertical gutter communicates with the mold at the top of said mold, specifically at the reservoir known as the feed head.
In this manner, as the mold is filled the main feeding gutter is blocked and thus the molten metal reaches the feed head through the secondary vertical gutter, after which the secondary channel is also blocked for a simple recovery under gravity of the molten metal which remains in the secondary gutter and in the main inlet gutter.
Filling of the mold and the feed head is controlled by a probe placed at the top area, which probe may move, i.e. it is retractable, so that it may occupy three positions, two corresponding to a greater or lesser insertion in the top of the mold to detect its filling and that of the feed head, and a third fully retracted position to allow insertion and removal of the sand mold, such that the probe sends the corresponding signals to the devices which seal the main and secondary casting gutters, causing their activation and the ensuing sealing of these gutters.
The sealing device is the same in both cases, although they are independent, and in each case it is opposite the corresponding gutter, in one case opposite the main casting gutter and in the other opposite the secondary gutter for films of the feed head. The device consists of a channel closing strip blade which cuts the sand of the mold and the casting gutter exactly when the mold is filled, without any displacement of sand.
The strip blades, instead of being mounted on the mold as in traditional systems, are mounted on an automatic loading clip outside the mold which can be placed under the machine itself or on its side, somewhat inclined to allow the ejection of each strip blade, with said ejection performed by the impulse of an element which is part of the piston of a pneumatic and hydraulic cylinder, preferably an impact cylinder, which comprises an impulse system which drives the piston or head and with it the element which in turn must push the strip blade at the front of the loading clip, so that said strip blade crosses the sand of the mold in the casting gutter and thereby seals it automatically.
Naturally, strip blades in the loading clip are constantly pressed on by an internal elastic element such as a spring which ensures that the first strip blade is always opposite the outlet of the clip.
Strip blades are as thin is possible, so that thee resistance to their penetration in the sand is small, and thus to improve their stiffness they preferably have an angular jagged profile either wavy or rectangular, which further allows use of thin plates and very sturdy and efficient impact mechanisms which improve penetration.
When the casting is performed with aluminium blades will preferably be of the same material, so that the aluminium alloy is not contaminated when recasting the return of the casting gutters. If a casting material is used with a higher melting, point the strip blades will preferably be made of steel.
The operational stages of the system of the invention may be summarised as follows:
The casting furnace is approached to the mold and the probe is placed in its position of maximum insertion in the mold.
The furnace is pressurised an( the mold filled with molten metal through the main gutter or inlet, until the level inside the mold reaches the height set by the probe. At the same time the molten metal in the secondary gutter reaches the same height as inside the mold.
The casting gutter or main inlet is closed or sealed with the corresponding sealing device.
The probe rises until it reaches the second measurement position, corresponding to the top level of the feed head.
Feeding of molten metal continues, only through the secondary gutter as the main feeding gutter has been previously sealed, until the molten metal reaches the level of the new position of the probe.
The probe generates a signal to cut off supply of the metal, activating the cut-off and sealing of the secondary gutter by the corresponding sealing device, which as mentioned before is identical to that which seals the casting gutter or main inlet.
The furnace is depressurised so that the liquid metal which remains in the main and secondary casting gutters returns to the furnace. Simultaneously the probe exits the furnace and occupies its fully retracted position so that the mold may be changed.
The furnace recedes.
A new mold advances.
A new cycle begins.
With this method and the operational stages described above, parts are obtained with a better quality than those obtained by traditional systems, as the hottest and therefore more liquid metal is at the top of the mold, and thus feeding from the feed head is improved and the volume and weight of the feed head is reduced.
Furthermore, the ratio of the total weight of metal used to the weight of the past obtained improves considerably.